This invention relates generally to radiant energy wave communication, and more particularly, to code modulation, correlation and demodulation in spread-spectrum communication systems.
A spread-spectrum system develops process gain in a sequential-signal bandwidth spreading and despreading operation. The transmit portion of the process may be accomplished with any of the band-spreading modulation methods, as for example, direct sequence modulation wherein the carrier is modulated by a digital code sequence having a bit rate which is much higher than the information signal bandwidth. The receive portion of the process is accomplished by despreading or correlating the received spread-spectrum signal with a locally generated reference signal having the same digital code sequence. When the two coded signals are matched, the desired signal elements, i.e., the data communicated, collapse to their original bandwidth (prior to spreading); and concurrently, any unmatched input is spread by the local reference to its bandwidth or more. A filter then rejects all but the desired narrow-band signal. Thus, given a received signal and its interference (atmospheric noise, receiver noise, and jamming signals), a spread-spectrum receiver enhances the signal while suppressing the effects of all other inputs.
Spread-spectrum signals are demodulated in two steps: first, the spectrum-spreading modulation or digital code sequence is removed; and second, baseband recovery is effected by demodulating the remaining signal which carries information by conventional modulation such as frequency modulation, frequency shift keying or phase shift keying. The present invention finds use in the first of these two steps.